Antifuses are devices that are used in integrated circuits (IC's) to provide various circuit selection and configuration functions. When fabricated as part of IC memory devices, antifuses are used, for example, to control access to pre-programmed applications and stored data.
An antifuse device is said to be programmed, or “blown,” when the antifuse device is altered from a high resistance state to a low resistance state. This is typically accomplished by applying a higher than normal voltage or a laser beam to the antifuse device. The higher voltage, for example, produces a low resistance antifuse circuit where a high resistance un-programmed antifuse circuit previously existed. A laser beam creates a similar low resistance circuit condition in the antifuse device. In digital circuit applications, a blown antifuse device changes a high impedance path to a current conducting path that effects a change in a logic level with an applied current. An antifuse structure generally comprises two conductors, either metal and/or a semiconductor material, that have some type of dielectric or insulating material between the two conductors. Previously, the dielectric or insulating material was in the form of a thin layer of high resistance amorphous silicon to which a programming voltage was applied to change the amorphous silicon into a thin layer of lower resistance polysilicon. Recently, the dielectric or insulating material is provided, for example, as a thinner-than-normal gate dielectric layer of an FET. In the presence of high voltage or laser power the thin oxide is electrically broken down and the antifuse device is programmed, or is said to blow. Typically, the change in conduction of an antifuse device is effected without substantially affecting any remaining components of a circuit.
In memory devices, antifuses are used, for example, in programmable read-only memory circuits. A device manufacturer may, for example, program proprietary information into a memory device and then blow one or more antifuses to deny access to the proprietary information. Antifuse cells are often used to disable particular functions in an IC after testing or to disable access to specific memory locations within a memory circuit.
Antifuse cells or circuits often receive multiple control signals, for example, a select signal (SEL), an enable signal (EN), a program voltage control signal (Vblow), etc. The corresponding terminals for each of these control signals create possible access points to the antifuse cell for an unauthorized user or for an intruder.
An input terminal for a programming voltage, or a Vblow signal terminal, for example, is particularly vulnerable to unauthorized access or intrusion. This is because, if a sufficiently high voltage is applied to the Vblow terminal, the input characteristics of an antifuse sense circuit may be altered such that an output signal of the protected circuit erroneously indicates that the antifuse is intact, or not programmed. A current method of preventing access to the Vblow input terminal is to route the Vblow signal to a core of an IC die from a pin or terminal that is not located on the IC die itself but is on the IC wafer for that IC die. According to this method, a die is tested before the die is parted from the IC wafer and the antifuse cell is appropriately programmed. After partition of the die from the wafer, the antifuse cell itself cannot be accessed. A drawback of this method of preventing access to the Vblow input terminal is that a die purchaser is prevented from programming the antifuse after the die is cut from the wafer.
What is needed is an antifuse cell that requires a minimum of control signals and that also allows a user to program an antifuse cell after the die has been cut from a wafer, while still providing a high level of protection against unauthorized access to the antifuse cell.